Ginnosuke Isuzu is a central character from the anime Tokyo Underground. He is a high school student and the best friend of the series' protagonist, Rumina Asagi, having been close to him since childhood. Physically, Ginnosuke is recognized as a teenage boy with blue eyes and black hair that he wears in a ponytail, with the length reaching his chest. He is almost always seen wearing a traditional gakuran school uniform and his most defining accessory is a pair of very thick glasses. In fact, his appearance is so drastically changed by his eyewear that his own friends often cannot recognize him when he removes them.

In terms of personality, Ginnosuke is portrayed as highly intelligent, possessing a sharp and analytical mind that complements Rumina's more straightforward and action-oriented nature. He is the brains of the duo, often using his intellect to assess situations and devise plans. However, his intelligence is largely bookish and academic, and he notably lacks what one would call street smarts, which can lead to him being caught off guard by the harsh realities of the world around him. His general demeanor is somewhat timid and cautious, displaying a great deal of common sense that often makes him the voice of reason when bolder characters rush into danger. Despite his usual hesitance, he is capable of becoming surprisingly fierce and determined when the situation absolutely demands it, showing a hidden reservoir of courage.

Ginnosuke's primary motivation stems from his unwavering loyalty to his friend Rumina. He does not possess any inherent special elemental abilities, a fact that sets him apart from the many "Elemental Users" from the Underground world. This lack of power does not deter him from participating in the dangerous journey, as his role in the story is to provide crucial non-combat support. He elects to follow Rumina into the world beneath Tokyo, known as the Underground, after their acquaintance Ruri Sarasa is kidnapped. Here, his intellectual contributions become his greatest asset. Reluctant to be a liability, he later utilizes a technologically advanced weapon capable of firing elemental attacks, allowing him to contribute to battles despite his lack of innate powers.

His relationships are central to his character arc. His bond with Rumina Asagi is the cornerstone of his actions, and his decision to travel into a dangerous, unknown world is driven entirely by his desire to aid his friend. He also develops key alliances with other characters from the Underground. Notably, he builds a rapport with a rebel named Sui, an Aerogun user who, after Ginnosuke saves him from agents of the Company, begins to refer to him as "Master Sui". Later in the series, after a series of events lead him and others to fall into the lawless slums of the Underground, Ginnosuke takes on a leadership role. He organizes and leads a team of allies, including Emily Ronolf, Jilherts Micheat, and Sui, to compete in a tournament held in the slums. This demonstrates a clear development from a timid, bespectacled sidekick into a capable and respected leader who can rally others to a common cause.